1. Technical Field
The present invention relates to microchip assembly. In particular, the present invention relates to an assembly comprising a substrate, a chip mounted on the substrate, and a voltage controlled oscillator circuit, and a method for fabricating the same.
2. Background Information
In the cellular telephone market consumers are demanding an increase of functionality, versatility and usefulness, while at the same time expecting cellular telephone costs to keep decreasing. Electronic devices like cellular telephones utilize a combination of radio frequency (RF) circuits and digital electronics. The RF circuitry is now being integrated into the same chip or a set of chips in order to present the consumer with a compact, light weight, and powerful electronic device.
The effective combination of RF functionality in mainstream digital CMOS technology is the key factor for developing low-power and cost-effective solutions for portable and cellular application devices. One of the crucial RF building blocks to integrate are the high-frequency oscillators, since they normally contain inductors, in particular inductor coils, and their performance depends strongly on the quality factor of the inductors. Over the last years the on-chip integration of the coil has succeeded, achieving the tight phase noise specification for various standards like the Digital European Cordless Telephone (DECT) standard, the Bluetooth standard and even the Global System for Mobile communications (GSM) standard. Today, in most CMOS technologies high quality inductors are provided by processing a thick top Cu layer at the expense of an additional non-standard back-end of line (BEOL) fabrication process. The additional fabrication process is however very expensive. The quality factor of the inductor is determined by the losses in the metal layers and at higher frequencies by the resistive losses in the substrate.
FIG. 1 shows the quality factor of a 6.8 nH integrated inductor on a silicon p− substrate. At lower frequencies, the quality factor is determined by the losses of the metal layers achieving a maximum quality factor Q≈6 at approximately 2 Gigahertz. At higher frequencies the quality factor decreases rapidly due to the losses in the silicon substrate. In a GSM RF transceiver the RF oscillator usually operates at 3.8 GHz and by dividing the signal by 2 and 4, respectively, the LO signal is generated depending whether GSM low-band (GSM850/900) or GSM high-band (PCS/DC1800) is chosen. Therefore, achieving high quality factors at about 3.8 GHz is of crucial importance to meet low-phase noise requirements and low-power consumption in an LC-VCO.